1. Field of the Invention
The invention is in the field of electrical power production from high temperature geothermal brines which have high dissolved chloride salt content.
2. Description of the Prior Art
There are certain geothermal resources that have been discovered in the world which contain large amounts of geothermal energy, but which have heretofore not been usable for the commercial production of electrical power because of a very high dissolved chloride salt content, and also because of a silica content which is proximate saturation at the high source temperatures. Such a geothermal resource is the lower geothermal reserve portion of the Salton Sea geothermal field in the Imperial Valley in California, wherein the geothermal brines produced below a depth of approximately 3,000 feet have very high source temperatures ranging from about 590.degree. F. to about 675.degree. F., contain from about 300,000 to about 360,000 parts per million by weight of chloride salts after the brine has been flashed to atmospheric pressure, with an average chloride salt content when flashed of approximately 325,000 parts per million, and contain silica proximate the saturated level in amounts of from about 550 to 600 parts per million by weight.
Although the lower reserve portion of the Salton Sea geothermal field is believed to be the largest single geothermal energy reserve in the world because of its very high temperatures, and the relatively large geographical area that it covers, and also because of its regenerative capacity, nevertheless commercial development of this resource has heretofore been considered impractical because flow tests from this resource have revealed that large amounts of sodium chloride and some potassium chloride would precipitate out as a hard, rock-like scaling in various parts of an electrical generating plant where temperature and pressure drops would occur, such as in production piping, steam separators, vessels, conduit joints, valves, instruments, and reinjection factilities. In a typical geothermal electric power generating plant, the geothermal brine would be cooled to approximately 212.degree. F. after its thermal energy had been extracted in the plant and brine was conducted to reinjection equipment, and the maximum solubility of the chloride salts in this brine is only approximately 250,000 parts per million at this reinjection temperature. Accordingly, with the original chloride salt content averaging about 325,000 parts per million chloride salts, there would be an excess of about 75,000 parts per million chloride salts above the soluble amount at reinjection temperatures, of which the approximately 40,000 parts per million of sodium chloride could potentially precipitate out in an electric generating plant powered from this geothermal source.
To put into perspective just how serious this problem could be, preliminary engineering studies by Magma Power Company of Los Angeles, Calif. for a 20 megawatt plant utilizing geothermal brine from this lower geothermal reserve portion of the Salton Sea geothermal field indicate that the plant will utilize approximately 3,000,000 pounds of brine per hour. If sodium chloride in an amount of approximately 40,000 parts per million of this flow were to precipitate out in plant and reinjection equipment, that would amount to approximately 120,000 pounds per hour of precipitated chloride salts, which would, of course, cause severe plugging and disposal problems.
In addition to this plugging problem, the high chloride salt content of the lower Salton Sea geothermal reserve brines also presents a corrosion problem to plant equipment. This problem is particularly severe at the high temperatures of the production brine as it flows upwardly through the conventional cemented-in production casing of a geothermal well. In fact, experience of applicants indicates that the salinity of fluids flowing up through conventional carbon steel casings must be of a much lower order of magnitude, preferably not more than about 50,000 parts per million, at these high temperatures in order for such casings to have a satisfactory life expectancy.
Temperature reductions as thermal energy is extracted in a geothermal electric power generating plant are also accompanied by a reduction in the silica saturation amount in the brine, which tends to cause a silica scaling problem in plant and reinjection equipment, in addition to the chloride salt scaling problem referred to above.
Some attention was given to the matter of controlling mineral precipitation in connection with a geothermal electrical power plant in U.S. Pat. No. 3,757,516 issued to Barkman C. McCabe. That patent taught the principle of deep well pumping in a geothermal hot water production well and pressurization throughout the entire plant system on through reinjection primarily to avoid the loss of the heat of vaporization from that portion of the fluid which would otherwise flash to steam in the production well, but also to prevent mineral precipitation at any point in the entire flow path. However, the said McCabe U.S. Pat. No. 3,757,516 was concerned only with geothermal plants that produced electrical power from geothermal resources having temperatures under about 400.degree. F., where the principal scaling problem involved the precipitation of carbonates in the production well bore and heat exchangers. Said McCabe U.S. Pat. No. 3,757,516 was not concerned with a very high temperature geothermal brine resource such as the lower reserve of the Salton Sea geothermal field, wherein the geothermal production brine has an extremely high chloride salt content that is much higher than the saturation level for reinjection temperatures, and also has a high, generally saturated silica content at production temperatures.
U.S. Pat. No. 4,043,129 issued to Barkman C. McCabe and Edward Zajac applied the deep well pumping concept of the earlier McCabe U.S. Pat. No. 3,757,516 to high temperature geothermal brines above about 500.degree. F. The McCabe-Zajac U.S. Pat. No. 4,043,129 taught that the advantages of deep well pumping, including both avoidance of the loss of heat of vaporization and minimization of mineral precipitation, could be realized in connection with very high temperature geothermal brines by mixing a high temperature geothermal brine which might have a relatively high mineral content derived from a relatively deep well with a lower temperature geothermal brine that might have a relatively lower mineral content derived from either a shallower geothermal well or a peripheral geothermal well, to produce a brine mixture of sufficiently lowered temperature to be within the practical temperature and pressure ranges for pumping, and would incidentally have a lowered mineral proportion. The minerals with which the said McCabe-Zajac U.S. Pat. No. 4,043,129 was principally concerned were, as with the earlier McCabe U.S. Pat. No. 3,757,516, carbonates. Regarding the chloride salt content of the brine, and incidentally the silica content thereof, said McCabe-Zajac U.S. Pat. No. 4,043,129 simply says (at column 18, lines 16-23) that where the chloride salt content of the geothermal brine is below about 250,000 parts per million, it is generally not too corrosive for use in available heat exchangers, provided the silica content of the fluid is sufficiently low. Then, said McCabe-Zajac U.S. Pat. No. 4,043,129 teaches (at column 24, line 66 to column 25, line 7) that where a geothermal brine is too corrosive or has too high a silica content for liquid-to-liquid heat exchangers, this can be overcome by flashing the fluid to steam in steam separators of a type that can be easily cleaned out on a periodic basis, and then the heat energy from the steam used for generating power.
Thus, the prior McCabe-Zajac U.S. Pat. No. 4,043,129 recognizes that if there is too much chloride salt content in a high temperature geothermal brine, then there will be corrosion and mineral fallout problems, and recognizes that the method taught therein of mixing the high temperature brine of a deep geothermal production well with the lower temperature brine of a shallower or peripheral geothermal production well does not solve either the chloride salt or the silica problem. To the contrary, it would be impractical to attempt to solve the problem of the very high chloride salt and silica contents of a high temperature geothermal resource like the lower Salton Sea geothermal reserve by mixing the brine from that resource with geothermal brine from an upper or peripheral reserve which may have lesser mineral content, for a number of reasons. First, in order to provide such upper or peripheral reserve geothermal brine, it would be necessary to drill and case a well to considerable depth, at considerable expense. Second, in order to prevent flashing and consequent plugging in such upper or peripheral resource well, it would be necessary to employ down-hole pumping, at considerable expense in both equipment and power consumption. Two upper reserve wells drilled by Magma Power Company in the Salton Sea geothermal field at approximately 1,500-foot and 2,100-foot depths produced geothermal brines with respective chloride contents of approximately 170,000 parts per million and 220,000 parts per million. Such huge amounts of upper reserve brines of this character would be required to be combined with a lower reserve brine having an average of about 325 parts per million chloride salt content to reduce the average chloride salt content to below about 250,000 parts per million that pumping equipment and plant size would become much too large to be practical. Even then, the highly corrosive character of such upper reserve brines injected in the annulus between the production casing and pump string according to said McCabe-Zajac U.S. Pat. No. 4,034,129 would seriously limit the operational life of not only the high temperature lower reserve well, but also of the lower temperature upper reserve well. It is estimated that if such dilution by an upper zone geothermal brine were employed, the lower section of the production casing of the lower reserve well would be completely eaten out by corrosion, and that section of the well would have to be redrilled and recased at least about every five years.